Problem: In history class, the girl to boy ratio is $2$ to $7$. If there are a total of $36$ students, how many boys are there?
A ratio of $2$ girls to $7$ boys means that a set of $9$ students will have $2$ girls and $7$ boys. A class of $36$ students has $4$ sets of $9$ students. Because we know that there are $7$ boys in each set of $9$ students, the class must have $4$ groups of $7$ boys each. There is a total of $28$ boys in history class.